Geology Reference
In-Depth Information
propagates in the direction opposite to the oxidizer flow, and in forward smolder, the front propagates in the
same direction.
These two configurations are distinguished by the roles played by heat and mass transport mechanisms and
chemical reactions. In forward propagation, the oxygen flows through the char, reacts at the smolder zone
and then the oxygen-depleted gas flow continues through the virgin fuel. Convective heat transport is
towards the virgin fuel ahead and results in preheating. In opposed propagation, the oxygen flows through
the virgin fuel and reacts at the smolder zone. Then the oxygen-depleted gas flow travels through the burned
region where char is. Convective transport is toward the burned fuel left behind the front, reducing the
preheating of the fuel.
Structure of a Smoldering Front
W
hen a smoldering front has been initiated and is propagating laterally, generally speaking, there are four
discernable regions propagating. As one moves toward the front from unburned fuel, these are:
1. Preheating of the undisturbed fuel: heat from the reacting front is transported ahead preheating the fuel up
to temperatures where water evaporation takes place. This front does not emit gases in any significant
quantity.
2. Evaporation: this endothermic reaction occurs at a significant rate within the range of temperatures from ~80 to
100°C, emitting water vapor. In this front the mass loss depends on the moisture content.
3. Burning region: this front is where the pyrolysis and oxidation reactions take place and net heat is released.
Pyrolysis reaction absorbs heat and converts the fuel into volatile gases, polyaromatic hydrocarbons, CO, CO
2
,
and water vapor. It leaves behind a solid carbonaceous char or coke. Pyrolysis starts approximately at
temperatures above 200
250°C. Subsequent heating above this temperature increases the pyrolysis rate if
fuel is available. Pyrolysis is stronger at deeper layers of the fuel where oxygen transport is reduced and thus
oxidation rate is lower. The oxidation involves the exothermic reaction of the fuel left by the pyrolysis front. The
peak temperature is found in this region and is where most of the fuel mass is lost. This reaction overlaps with
the pyrolysis depending on the propagation mode and oxygen availability. The oxidation reaction occurs at
temperatures over 300°C and is the main source of CO and CO
2
. More CO
2
is formed where the oxygen supply
is large (e.g., closer to the oxygen supply or free surface) and more CO where it is limited (e.g., further from the
oxygen supply or deeper into the fuel layers).
4. Char and ash region: this is where the smoldering has ceased and the remaining matter cools to ambient
temperature. The ash left is the mineral content present in the original fuel and the char or coke is the result of
incomplete burning.
-
The propagation rate of self-sustained smoldering is typically controlled by oxygen transport and net heat losses.
Yet, heterogeneous chemical kinetics governs the front structure and dictates the effective value of the global heat
released. The degradation of a solid fuel involves multiple pathways to chemical changes (pyrolysis and oxidation),
and these pathways are not yet fully understood. In spite of the complex kinetic behavior, experimental evidence
suggests that mechanisms consisting of only a few global reactions capture the most important characteristics of the
decomposition process. The polymers for which smoldering kinetics are best known are cellulose (Kashiwagi and
Nambu, 1992) and polyurethane foam (Rein et al., 2006).
In forward smoldering propagation, the oxidation and the pyrolysis reactions form two distinct propagating fronts.
The pyrolysis front arrives first to the virgin foam and then followed by the oxidation front. This is in agreement
with experimental measurements of forward propagation where two distinct fronts are observed in the temperature
profiles. In opposed smoldering, the oxidation and the pyrolysis reactions overlap to form a single propagating
front. This is also in agreement with experimental observations in opposed propagation where a single front is
observed in the temperature profiles.
Figure 17.1.4 shows the one-dimensional representation of a smoldering front in a fuel rod and the approximate
correspondence with a burning cigarette (one of the most common examples of smoldering). As shown by the
results in Figure 17.1.4 for forward smoldering, the pyrolysis front is located at the leading edge of the cigarette
burning front since it does not need oxygen to permeate into the solid. The oxidation reaction takes place at the
